Synthesis-surface-modification-and-photocatalytic-property-of-ZnO-nanoparticles Powder-Technology

Synthesis,surface modi ?cation and photocatalytic property of ZnO nanoparticles

R.Y.Hong a ,b ,?,J.H.Li a ,L.L.Chen a ,D.Q.Liu a ,H.Z.Li b ,Y.Zheng c ,J.Ding d

Chemical Engineering Department &Key Laboratory of Organic Synthesis of Jiangsu Province,Soochow University,SIP,Suzhou 215123,China b

State Key Laboratory of Multiphase Reaction,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100080,China c

Department of Chemical Engineering,University of New Brunswick,Fredericton,N.B.,Canada E3B 5A3d

MSC Software,2MacArthur Place,Santa Ana,CA 92707,USA

a b s t r a c t

a r t i c l e i n f o Article history:

Received 23December 2007

Received in revised form 9May 2008Accepted 18July 2008

Available online 31July 2008Keywords:

ZnO nanoparticles Surface modi ?cation Graft polymerization

Photocatalytic degradation Methyl orange

ZnO nanoparticles were synthesized by calcination of precursor prepared by the precipitation method.Polystyrene was grafted onto the surface of ZnO nanoparticles to improve the dispersion of the particles and to reduce their photocatalytic activity.The obtained particles were characterized by Fourier transform infrared spectroscopy,X-ray powder diffraction,and transmission electron microscopy.The photocatalytic activity of bare and modi ?ed ZnO nanoparticles was studied.The in ?uence of surface modi ?cation on the photocatalytic degradation of methyl orange has been analyzed.The composition of residual solution was determined through high performance liquid chromatography.Experimental results show that well dispersed ZnO nanoparticles were obtained after surface modi ?cation.ZnO nanoparticles possess high photocatalytic activity,whereas the photocatalytic activity can be signi ?cantly reduced when polystyrene was grafted onto the particle surface.

1.Introduction

The nanosized ZnO with the features of large volume to area ratio,high ultraviolet (UV)absorption,and long life-span [1]has been widely used as catalyst [2,3],gas sensor [4,5],active ?ller for rubber and plastic,UV absorber in cosmetics and anti-virus agent in coating [6–8].

ZnO nanoparticles can be synthesized by various approaches including sol –gel processing [9,10],homogeneous precipitation [11],mechanical milling [12],organometallic synthesis [13],microwave method [14],spray pyrolysis [15,16],thermal evaporation [17]and mechanochemical synthesis [18].However,ZnO nanoparticles are prone to aggregate due to the large surface area and high surface energy.In order to improve the dispersion,it is necessary to modify the surface of ZnO nanoparticles.Some researches have revealed several physical and chemical methods for modifying the surface of ZnO nanoparticles.The chemical surface modi ?cation,which can be classi ?ed as surface grafting and esteri ?cation,is the most promising method because of the strong covalent bond between the surface modi ?ed particles and polymer chains.

It is reported that some semiconductor,such as nanosized TiO 2or ZnO,has attracted extensive attention as a photocatalyst for the degradation of organic pollutants in water and air under UV irradiation [19–23].Photocatalyst is also called photochemical catalyst and the function is similar as the chlorophyll in the photosynthesis.In a

photocatalytic system,photo-induced molecular transformation or reaction takes place at the surface of the catalyst.A basic mechanism of photocatalytic reaction on the generation of electron –hole pair and its destination is as follows:when a photocatalyst is illuminated by the light stronger than its band gap energy,electron –hole pairs diffuse out to the surface of the photocatalyst and participate in the chemical reaction with the electron donor and acceptor.Those free electrons and holes transform the surrounding oxygen or water molecules into OH U free radicals with super strong oxidization [24].However,for the application of ZnO nanoparticles in cosmetic,the OH U free radicals are harmful to human beings due to the super strong oxidization of OH U free radicals generated on the surface of ZnO nanoparticles.Therefore,it is necessary to modify the surface of ZnO nanoparticles to obtain well UV shielding ability.

In our previous research,the ZnO nanoparticles were modi ?ed by SiO 2[23],PMMA [25]and PSt [26],and the in ?uence of particles on the mechanical properties of polymer matrix was studied.In the present investigation,ZnO nanoparticles were ?rst treated with the coupling agent KH-570,and then PSt (polystyrene)long chains were anchored on the surface of ZnO nanoparticles through grafting polymerization.The dispersion and photocatalytic activity of the modi ?ed ZnO nanoparticles were investigated.2.Experiments 2.1.Materials

Polyethylene glycol (M w =4000,AR),Zn(CH 3COO)2·2H 2O (AR),(NH 4)2CO 3(AR),ammonia solution (AR),anhydrous ethanol (AR),

Powder Technology 189(2009)426–432

?Corresponding author.Chemical Engineering Department,Soochow University,SIP,Suzhou 215123,China.Tel.:+8651266000797;fax:+8651265880089.

E-mail address:rhong@https://www.360docs.net/doc/947848858.html, (R.Y.

10.1016/j.powtec.2008.07.004

Contents lists available at ScienceDirect

Powder Technology

j o u r n a l h o me p a g e :w w w.e l sev i e r.c o m /l oc a t e /pow t e c

ammonium iron(II)sulfate hexahydrate(AR),silver sulfate(AR), potassium dichromate(AR),methyl orange(C14H14O3N3SNa,Q/ HG22-2406-90),1,10-phenanthroline(C12H8N2·H2O,AR),methyl red(C15H15N3O2,HG3-958-76),phenolphthalein(C20H14O4,Q/CYDZ-04-92),xylene(AR),acetone(AR),tetrahydrofuran(THF,AR),styrene (AR)and azobisisobutyronitrile(AIBN,AR)were all purchased from the Sinopharm Chemical Reagent Co.,Ltd.KH-570silane coupling agent(CP)was purchased from Shanghai Yaohua Chemical Reagent Co.,Ltd.

2.2.Preparation

2.2.1.Preparation of ZnO nanoparticles

ZnO precursors were synthesized by the precipitation method and then calcined to obtain ZnO nanoparticles.Firstly,polyethylene glycol solution was syringed into a three-neck?ask.Then,Zn (CH3COO)2·2H2O and(NH4)2CO3aqueous solutions were dropped into the?ask at the same time with vigorous stirring.After reacting for 2h at room temperature,the precipitates were washed and?ltered with ammonia solution(pH=9)and anhydrous ethanol for several times,and dried under vacuum for12h.Finally,the precursors were calcined in an oven at450°C for3h and milled,and then ZnO nanoparticles were obtained.

2.2.2.Grafting PSt onto the surface of ZnO nanoparticles

Firstly,the surface of ZnO nanoparticles was activated by KH-570 silane coupling agent.Typical steps were given as follows:1.0g of ZnO nanoparticles,1.0mL of KH-570,and40mL of xylene were added into a four-neck?ask(ca.250mL)and re?uxed at80°C for3h under magnetic stirring and argon protection.After centrifuging and extracting with alcohol for12h to remove the residual silane,the precipitate was dried in vacuum for12h.From the above treatment, the double bonds were introduced onto the surface of the ZnO nanoparticles.

Secondly,the surface activated ZnO nanoparticles were grafted by PSt via a typical solution polymerization.The procedure is described as follows:1.0g of modi?ed ZnO nanoparticles and50mL of xylene were put in a?ask equipped with a re?ux condenser.When the temperature reached80°C,160mg of AIBN was added into the?ask, and then4mL of styrene was added by drip-feeding for30min under stirring and argon atmosphere.After3h,the resultant suspension was centrifuged and washed with acetone for three times.The dried precipitates were extracted with THF for24h to eliminate PSt homopolymers.Then,the grafted nanoparticles were dried under vacuum for24h.

2.3.Photocatalytic degradation procedure

Nanosized ZnO particle is a new photocatalyst to degrade organic contaminants,such as MeOr.The photocatalytic degradation of MeOr using nanosized ZnO is investigated with the following process:some ZnO nanoparticles were suspended into MeOr aqueous solution (about50mL),and the obtained suspension was kept in a dark environment with stirring for30min.Then,the suspension was syringed into a hollow cylindrical reactor made of glass,in which air was bubbling continuously from the bottom of the reactor.Sampling per hour and centrifuging,we measured the UV–vis absorption of the clari?ed solution at the wavelength from200nm to800nm.Finally, photocatalytic degradation percentage(PDP)of MeOr was calculated using Eq.(1),

PDP k?A0?A

?100ke1T

where,A0and A are the UV–vis absorption of original and sampled solutions,respectively.2.4.Characterization

Surface structure of all samples was characterized by a Nicolet Avatar360Fourier transform infrared(FT-IR)spectroscope.Measure-ments were performed with pressed pellets made using KBr powder as diluent.The FT-IR spectrum was collected between the wave number of400and4000cm?1.

The bare and modi?ed ZnO nanoparticles were characterized by X-ray diffraction(XRD)(D/Max-IIIC,Japan)using Cu-Kαradiation (λ=1.5406?).Distances between peaks were compared to the JCDPS 5-0664of the International Center for Diffraction Data to determine crystalline structures.

The morphology of the bare and modi?ed ZnO nanoparticles was determined by transmission electron microscope(TEM,Hitachi H-600-II)with an acceleration voltage of200kV.The samples for TEM analysis were prepared by dropping dilute suspension of ZnO nanoparticles onto copper meshes.

The Perkin-Elmer TGA-7with variable temperatures from0°C to 700°C was employed to determine the coverage percentage of polystyrene on the surface of ZnO nanoparticles.The test was performed in nitrogen atmosphere with the accelerated speed of 20°C/min.

The dispersibility of the modi?ed nanoparticles was characterized by the sedimentation test.Typically,the modi?ed nanoparticles (30mg)and acetone(30mL)were charged into a scale test tube with a plug,and the tube was required to stand immovably at room temperature.After a de?nite time,the depth of the suspension was recorded.The less the depth of the suspension is,the better the stability of the modi?ed particles is.The sedimentation percentage of modi?ed particles was determined by the following equation:the sedimentation percentage of modi?ed particles(%)=H/H0×100%, where H(cm)is the variable depth of the suspension and H0(cm)is the total depth of the suspension.

The size of the PSt-grafted ZnO particles/aggregates suspended in acetone was determined by Malvern HPPS5001dynamic light scattering with the scanning range of0.6to6000nm.The samples were prepared with sonicate before measurement.

The Delta320aciditymeter(Toledo,America)was used to modulate the pH of solutions.The UV-2812spectrophotometer (Hitachi,Japan)was used to measure the absorption of azo dyes solution.The high performance liquid chromatography(HPLC, Waters-510,America)was used to determine the composition of the residual solution.

The chemical oxygen demand(COD)was measured by dichro-mate method as follows:10.0mL of potassium dichromate solution (0.250mol/L)and10.0mL of dye solution before or after treatment were poured into a vessel with a re?ux apparatus.After switching on the cooling water,30mL of silver sulfate/sulfuric acid was poured into the vessel.Turn on the heater and keep the solution boiling for 2h.Wash the condensator with20mL of deionized water,dilute the solution to140mL,and cool it to room temperature.Ammonium iron (II)sulfate was used to measure the residual dichromate quantita-tively using1,10-phenanthroline as an indicator.The solution quickly turned from yellow to cyan,and to mahogany?nally indicating the end of titration.The volume of consumed ammonium iron(II)sulfate was recorded as V2.The COD was calculated according to Eq.(2),

COD mg=L

eT?

C V1?V2

eT?8000

e2T

where,C is the concentration of ammonium iron(II)sulfate,mol/L;V1 is the volume of consumed ammonium iron(II)sulfate in blank experiments,mL;and V0is the volume of a sample,mL.

Sampling per hour,we measured the UV–vis absorption at the wavelength between200and800nm.The characteristic absorption

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curve of MeOr aqueous solution was received,as shown in https://www.360docs.net/doc/947848858.html,paring the peak positions of those curves,one can ?nd that the peak position was blue-shifted along with reaction time,which is in

reasonable agreement with the literature [27]taking account of the experimental errors.3.Results and discussion

3.1.Characteristics of ZnO nanoparticles

3.1.1.FT-IR spectra

Fig.2a shows the FT-IR absorption spectrum of ZnO nanoparticles.The peak at 472.54cm ?1is the characteristic absorption of Zn –O bond and the broad absorption peak at 3438.26cm ?1can be attributed to the characteristic absorption of hydroxyls.It is also found that the infrared spectrum of the coupling agent (KH-570)treated ZnO nanoparticles exhibits absorptions at 2951.3,1716.8,1169.8,941.3and 817.9cm ?1as shown in Fig.2b,which could be ascribed to the characteristic peaks of –CH 2–stretching vibration,C f O stretching vibration,–SiOH stretching vibration,Zn –O –Si and Si –O –Si symmetrical stretching vibration,respectively.Such results indicate that the active groups have been introduced onto the nanoparticle surface.Fig.2c shows the FT-IR spectrum of the PSt-grafted ZnO nanoparticles.The absorptions at 1712.9,1631.9and 1454.4cm ?1,manifest the existence of PSt.Therefore,the polystyrene chains were successfully grafted onto the surface of ZnO

nanoparticles.

Fig.1.UV –vis spectrum of MeOr/ZnO suspension at different

times.

Fig.2.FT-IR spectrum of:(a)bare ZnO nanoparticles;(b)KH-570-grafted ZnO nanoparticles;(c)PSt-grafted ZnO nanoparticles.

428R.Y.Hong et al./Powder Technology 189(2009)426–432

3.1.2.X-ray diffraction

The XRD spectra of bare and PSt-grafted ZnO nanoparticles are shown in Fig.3.From Fig.3a,a series of characteristic peaks:2.814(100),2.608(002),2.475(101),1.911(102),1.624(110)and 1.478(103)are observed,and they are in accordance with the zincite phase of ZnO (International Center for Diffraction Data,JCPDS 5-0664).No peaks of impurity are observed,suggesting that the high purity ZnO was obtained.In addition,the peak is widened implying that the particle size is very small according to the Debye –Scherrer formula D =K λ/(βcos θ),where K is the Scherrer constant,λthe X-ray wavelength,βthe peak width of half-maximum,and θis the Bragg diffraction angle.The average crystallite size D is 20nm calculated using the Debye –Scherrer formula.

Fig.3b illustrates that after modi ?cation,the characteristic peaks have broadened and the characteristic peaks are still in accordance with the zincite phase of ZnO,indicating that the grafted polymer does not in ?uence the crystalline structure.

3.1.3.Transmission electron microscope

The transmission electron micrographs (TEM)of the bare and PSt-grafted ZnO nanoparticles are shown in Fig.4.Fig.4a shows that most of the bare ZnO nanoparticles are quasi-spherical and their diameter is about 20nm.This result is in accordance with the value calculated from the X-ray diffraction.Due to large speci ?c surface area and high surface energy,some nanoparticles aggregate.The aggregation occurred probably during the process of drying.Fig.4b shows the TEM of the surface modi ?ed nanoparticles,illustrating that aggrega-tion was alleviated and dispersion was improved.Besides,one can also observe that around the ZnO nanoparticles there encircle some shadows,implying the existence of the grafted polymer layers.3.1.4.TG analysis

Fig.5shows the TG curves of the bare and PSt-grafted ZnO nanoparticles,and the temperature scale for the measurement is from 50°C to 700°C.From Fig.5a,one can ?nd that about 4.6%of the total weight of bare ZnO nanoparticles is lost.The weight loss of bare ZnO nanoparticles may be due to the evaporation of water adsorbed on the surface of ZnO nanoparticles.Fig.5b is the TG curve of PSt-grafted ZnO nanoparticles and reveals that about 83.3%of the total weight of PSt-grafted ZnO nanoparticles is reserved.For PSt-grafted ZnO nanopar-ticles,the weight loss resulted from the evaporation of adsorbed water and the burn-up of grafted https://www.360docs.net/doc/947848858.html,paring the two TG curves,one can deduce that about 12.1wt.%of the total weight loss of PSt-grafted ZnO nanoparticles is attributed to the grafted polystyrene.3.1.5.Sedimentation test

To check the effect of surface modi ?cation,the dispersibility of PSt-grafted ZnO nanoparticles in acetone was compared with that of bare ZnO,as shown in Fig.6.Results indicate that bare ZnO nanoparticles completely precipitated after 4h,while PSt-grafted ZnO nanoparticles formed a stable dispersion in acetone.This illustrates that the surface modi ?cation can increase the compat-ibility between ZnO nanoparticles and organic solvents.This also shows that the grafted polymer chains on ZnO nanoparticles can result in the steric repulsive forces among particles,preventing the ZnO nanoparticle from aggregation.In addition,the hydrophilicity of the nanoparticle surface changes to extreme hydrophobicity.3.1.6.Particle size distribution

The particle size distribution of PSt-grafted ZnO nanoparticles in acetone suspension is shown in Fig.7.The number of d (0.99)num is 32.07nm,which means that the median size of 99%particles in volume is 32.07nm.The core size of PSt-grafted ZnO nanoparticles measured by TEM is about 20nm,which is smaller than that of

PSt-

Fig.3.XRD spectrum of:(a)bare ZnO nanoparticles;(b)PSt-grafted ZnO composite

nanoparticles.

Fig.4.TEM photographs of:(a)bare ZnO nanoparticles;(b)PSt-grafted ZnO nanoparticles.

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grafted ZnO nanoparticles in aqueous suspension measured by a laser particle size analyzer.The enlargement of particle size may be due to the grafted PSt layers.Besides,one can also ?nd that there is only one narrow peak that existed in the ?gure,implying that after graft polymerization well dispersed ZnO nanoparticles are obtained.3.2.Photocatalytic activity

3.2.1.Optimal conditions for photocatalytic degradation

A series of experiments were carried out to study the in ?uences of irradiation time,reaction temperature,pH,catalyst concentration,MeOr concentration and air ?ow rate on the photocatalytic degrada-tion percentage (PDP)of MeOr by ZnO nanoparticles.The results show that the PDP differs very much at different irradiation times and reaction temperatures,and the photodegradation reaction kinetics is approximately the ?rst-order.The degradation of MeOr is favored at higher temperature.However,when the reaction temperature is over 40°C,the solvent would easily evaporate with air bubbling.The optimal acidity for photocatalytic degradation reaction is at a pH of 7.0.When the pH is below 7.0,the PDP increases quickly with the increasing pH,otherwise,the PDP decreases.The PDP increases with the increasing catalyst concentration.However,at low concentration,the photodegradation is faster than that at high concentration.With

the increase of the MeOr concentration,the decolorization ef ?ciency decreases.Experiments performed at different air ?ow rates from 0to 100L/h indicate that the PDP increases from 43%to 90%with the increasing air ?ow.

Therefore,the optimal conditions for photocatalytic degradation of MeOr using ZnO nanoparticles are reacting for 5h at a MeOr concentration of 20ppm,a catalyst concentration of 1.5g/L,a pH of 7.0and a temperature of 30°C.

https://www.360docs.net/doc/947848858.html,parison of bare and PSt-capped ZnO nanoparticles

The PDP of MeOr solution after UV irradiation using different photocatalysts is illustrated in Fig.8.One can ?nd that the bare ZnO nanoparticles have much higher photocatalytic activity since almost 80%of MeOr is degraded after 4h.This may be due to the fact that bare ZnO nanoparticles are hydrophilic.They can absorb more MeOr molecules in water and can contact with air closely.While the PSt-capped ZnO nanoparticles are hydrophobic leading to little photo-catalytic activity.This can be explained that the long polymer chains coated on the surface of ZnO nanoparticles interfere with the absorption of the MeOr molecules and the contact with air.Moreover,the UV absorption of ZnO nanoparticles may be reduced due to the grafted PSt layers.Although the inner of the ZnO nanoparticles is quite active and can produce the electron –hole pairs under UV irradiation,the surface modi ?ed ZnO nanoparticles cannot adsorb enough MeOr molecules,and also the contact of PSt-grafted ZnO nanoparticles with air is obstructed by the grafted PSt layers.On the other hand,even if some electrons or holes are produced on the inner surface of ZnO nanoparticles,the electrons or holes cannot reach the outer surface of the modi ?ed particles,leading to the failure of photocatalytic degradation of MeOr.So,the PDPs of bare and PSt-grafted ZnO nanoparticles are entirely different under the same condition,and the photocatalytic activity of ZnO nanoparticles is greatly reduced after coating with

PSt.

Fig.5.TG curves of bare (a)and PSt-grafted (b)ZnO

nanoparticles.

Fig.6.Sedimentation of bare and PSt-grafted ZnO nanoparticles in

acetone.Fig.7.Size distribution of bare and PSt-grafted ZnO nanoparticles in

water.

Fig.8.PDP of MeOr using bare and PSt-grafted ZnO nanoparticles.

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3.2.3.UV –vis absorption

In order to prove the argument of which the PSt layers grafted on the surface of ZnO nanoparticles could reduce the photocatalytic activity of ZnO,the UV –vis absorption of bare and PSt-grafted ZnO nanoparticles was measured.Fig.9shows the UV –vis absorption curves of bare and PSt-grafted ZnO nanoparticles.It is found that the UV –vis absorption of ZnO nanoparticles is reduced after grafting PSt layers,especially at the wavelength from 250nm to 400nm.That is to say,the UV absorption of the modi ?ed ZnO is reduced after the surface is modi ?ed by PSt.It could at a certain extent explain the reduction of photocatalytic activity of PSt-grafted ZnO nanoparticles.Moreover,PSt-grafted ZnO nanoparticles were laid on a supporter under continuous irradiation of UV light for different time,and the UV –vis absorption measurement for the obtained PSt-grafted ZnO nanoparticles with different irradiation time was performed,as shown in Fig.10.From Fig.10,one can ?nd that at the ?rst 10h,the UV –vis absorption of PSt-grafted ZnO nanoparticles is almost the same,showing excellent UV light fastness,and then the UV –vis absorption increases.The increase of UV –vis absorption of PSt-grafted ZnO nanoparticles may be due to the destruction of PSt layers during the UV irradiation.

3.3.By-products analysis

In order to analyze the by-products during the photocatalytic degradation of MeOr,HPLC measurements were performed using water as ?ow phase.It is observed that there are another four peaks that appeared after MeOr solution irradiated by UV for 4h,contrast to the original MeOr solution,which indicates that there are mainly four new by-products generated.According to the reported article [28],the possible by-products are as

follows:

Besides,there are also some weak peaks of other low molecular weight by-products,which is well in accordance with the previous report [29].

3.4.Photocatalytic degradation of other dyes

The degradation of some other azo dyes,such as methyl red,phenolphthalein and 1,10-phenanthroline is also studied,and the results are listed in Table 1.From Table 1,one can ?nd that the PDP of methyl red,phenolphthalein and 1,10-phenanthroline is lower than that of MeOr,but the ratio of COD before and after photodegradation goes reversely,suggesting that the organic contamination of industrial wastewater can be evaluated by COD more accurately than PDP.4.Conclusions

ZnO nanoparticles with an average diameter of 20nm were obtained,and subsequently were grafted by PSt.The optimal conditions for photocatalytic degradation of MeOr using ZnO nano-particles are reacting for 5h at a MeOr concentration of 20ppm,a catalyst concentration of 1.5g/L,a pH of 7.0and a temperature of 30°C.The PDP increased with the increasing air ?ow rate.The photodegrada-tion of MeOr obeyed the rule of a pseudo ?rst-order reaction.The

bare

Fig.9.UV –vis absorption curves of bare and PSt-grafted ZnO

nanoparticles.

Fig.10.UV –vis absorption curves of PSt-grafted ZnO at different irradiation time under UV light.

Table 1

Photocatalytic degradation of some other azo dyes using ZnO nanoparticles

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ZnO nanoparticles showed high photocatalytic activity in photode-gradation of MeOr under UV irradiation,and mainly produced four by-products.The photocatalytic activity of ZnO nanoparticles for some other azo dyes was also very high,which showed a good application foreground in organic contamination treatment.Surface modi?cation of ZnO nanoparticles by polystyrene can improve the compatibility of the inorganic nanoparticles and the organic matrix,however,the photocatalytic activity of the modi?ed ZnO is reduced.

Acknowledgment

The project was supported by the National Natural Science Foundation of China(NNSFC,No.20476065and No.20736004),the Scienti?c Research Foundation for ROCS of the State Education Ministry(SRF for ROCS,SEM),the Key Laboratory of Multiphase Reaction of the Chinese Academy of Sciences(No.2006-5),the State Key Lab.of Coal Conversion,CAS(No.2006-902),the Key Laboratory of Organic Synthesis of Jiangsu Province,Foundation of Chemical Experiment Center of Soochow University and R&D Foundation of Nanjing Medical University(NY0586).

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